1. Field of the Invention
The present invention generally relates to a fixing unit and an image forming apparatus provided with the same. Specifically, the present invention relates to the fixing unit for the image forming apparatus such as a copying machine, a facsimile, a printer or the like, the fixing unit including a fixing rotatable body such as a fixing belt, a pressing rotatable body such as a pressing roller arranged abutting against the fixing rotatable body, and heaters for heating these rotatable bodies.
2. Description of the Related Art
Conventionally there is known a fixing unit provided for an image forming apparatus such as a copying machine, a facsimile, a printer or the like. The fixing unit includes a fixing rotatable body such as a fixing belt, a pressing rotatable body such as a pressing roller arranged abutting against the fixing rotatable body, and a heater for heating these rotatable bodies. And, there is known an image forming apparatus provided with such a fixing unit (for example, see Japanese Patent Application Laid-open No. 2009-139674 and Japanese Patent Application Laid-open No. 2007-079142).
In such a fixing unit, in order to achieve high-quality fixing, it is extremely important to keep the fixing rotatable body at a certain temperature suitable for fixing while fixing is being carried out. However, in a state that the fixing rotatable body, the pressing rotatable body and the like are not sufficiently heated, especially for example immediately after the image forming apparatus is turned on, if a recording medium such as paper is passed through the nip between the fixing rotatable body and the pressing rotatable body in order to fix a toner image on the recording medium, the recording medium, a toner carried on the recording medium, and the pressing rotatable body which is not sufficiently heated may absorb heat from the fixing rotatable body to decrease a temperature of the fixing rotatable body. Thereby, if several to several tens of paper sheets are passed continuously, the temperature of the fixing rotatable body may fall below the lower limit of the fixing temperature to deteriorate the fixing quality. Especially, even in a case that the fixing is performed only for one recording medium when the recording medium carries color toners, the temperature of the fixing rotatable body may decrease from a time when a leading edge of the recording medium passes through the nip to a time when a trailing edge of the recording medium passes through the nip. As a result, a glossiness of the fixed image has difference between the leading edge and the trailing edge, so that the leading edge has the lowered glossiness.
Therefore, in order to prevent or suppress the temperature drop of the fixing rotatable body, there is proposed a technology for providing a supplementary heater for the fixing rotatable body (for example, see Japanese Patent Application Laid-open No. 2009-139674), and a technology for providing a supplementary heater for the pressing rotatable body (for example, see Japanese Patent Application Laid-open No. 2007-079142). The reason why a heater is provided for the pressing rotatable body to suppress the temperature drop of the fixing rotatable body is that the pressing rotatable body is one of the structures that deprive the fixing rotatable body of heat. In other words, the technology aims to increase the amount of heat applied to the pressing rotatable body in order to reduce the amount of heat that the pressing rotatable body deprives from the fixing rotatable body.
In recent years, however, a thickness of the fixing rotatable body, as well as a size of rollers or the like which support the rotatable body when the body is a belt, becomes thinner in order to stop or suspend the heating during the standby mode and start the heating to immediately rise the temperature of the fixing rotatable body when starting the image forming, for the purpose of shortening a warm-up time of the fixing unit, saving the energy, reducing a TEC (Typical Electricity Consumption) value, and so on. As a result, the heat capacity is reduced. Thereby, the pressing rotatable body easily absorbs the heat from the fixing rotatable body to decrease the temperature of the fixing rotatable body, especially during the warm-up time. Therefore, the fixing rotatable body provided with the auxiliary heating unit still has a problem of deteriorating the fixing property.
This problem occurs even when a supplementary heater is provided for the pressing rotatable body. This is because it takes time for the heat generated by the heater to reach the external surface from the internal surface of the pressing rotatable body since the conventional heater is arranged inside of the pressing rotatable body and the pressing rotatable body has a certain thickness. In other words, the temperature gradient is formed in the thickness direction from the inside to the outside of the pressing rotatable body, depending on the thickness of the pressing rotatable body, and the thermal responsiveness is limited even if the pressing rotatable body is heated by the heater.
In order to save the energy, some studies have been conducted on toner that can be fixed at lower temperature. This technology could contribute to maintain the fixing property while the temperature is low at the time of a system startup. However, in this example, since the toner is prescribed to have a lower softening temperature, a so-called “hot offset” phenomenon may occur when the toner collected and attached to a cleaning roller or the like reaches a “flow start temperature” higher than the softening temperature while in use. As a result, the toner may be adversely re-transferred onto a member contacted with the cleaning roller, so that an abnormal image is induced. Therefore, it is desirable to ensure the fixing property during the warm-up time by the configuration of the fixing unit itself.